Retrieve list of taxonomies in json

I have custom taxonomies

like that

/wp-admin/edit-tags.php?taxonomy=my_cat&post_type=my_poet

I want retrieve list of taxonomies in json

add_action('init', array($this, 'json_handler'));

I tried

function json_handler(){
$categories = get_terms( 'my_cat', 'orderby=count&hide_empty=0' );
print_r($categories);
}

Outpot

WP_Error Object ( [errors:WP_Error:private] => Array ( [invalid_taxonomy] => Array ( [0] => Invalid taxonomy ) ) [error_data:WP_Error:private] => Array ( ) ) 

But it working for normal category

$categories = get_terms( 'category', 'orderby=count&hide_empty=0' );
    print_r($categories);

Outpot

Array ( [0] => stdClass Object ( [term_id] => 1 [name] => Uncategorized [slug] => uncategorized [term_group] => 0 [term_taxonomy_id] => 1 [taxonomy] => category [description] => [parent] => 0 [count] => 1 ) )

Here is my_cat register_taxonomy

add_action('init', array($this, 'create_stores_nonhierarchical_taxonomy'));
function create_stores_nonhierarchical_taxonomy() {
// Labels part for the GUI
$labels = array(
'name' => _x( 'Store  Categories', $this -> textdomain ),
'singular_name' => _x( 'Store  Categories', $this -> textdomain ),
'search_items' =>  __( 'Search Store  Categories',$this -> textdomain ),
'popular_items' => __( 'Popular Store  Categories',$this -> textdomain ),
'all_items' => __( 'All Store  Categories ',$this -> textdomain ),
'parent_item' => null,
'parent_item_colon' => null,
'edit_item' => __( 'Edit Store Categories',$this -> textdomain ), 
'update_item' => __( 'Update Store Categories' ,$this -> textdomain),
'add_new_item' => __( 'Add New Store Categories',$this -> textdomain ),
'new_item_name' => __( 'New Store Categories Name',$this -> textdomain ),
'add_or_remove_items' => __( 'Add or remove Store Categories',$this -> textdomain ),
'choose_from_most_used' => __( 'Choose from the most used Store Categories',$this -> textdomain ),
'menu_name' => __( 'Store Categories',$this -> textdomain ),
);
// Now register the non-hierarchical taxonomy like tag
register_taxonomy('my_cat',$this->post_type,array(
'labels'            => $labels,
    'hierarchical'      => true,
    'show_ui'           => true,
    'how_in_nav_menus'  => true,
    'public'            => true,
    'show_admin_column' => true,
    'query_var'         => true,
'rewrite' => array( 'slug' => 'my_cat' ),
));
}

Thanks